GrowList.cs

public class GrowList<T> {
    public T[] Items;
    public int Count;

    public GrowList (int capacity) {
        Items = new T[capacity];
        Count = 0;
    }

    public void Add (T item) {
        if (Items.Length == Count) {
            Array.Resize (ref Items, Items.Length << 1);
        }
        Items[Count++] = item;
    }

    public void EnsureCapacity (int count) {
        if (Items.Length < count) {
            var len = Items.Length << 1;
            while (len <= count) {
                len <<= 1;
            }
            Array.Resize (ref Items, len);
        }
    }
}